1. Field of the Invention
The present invention relates generally to Driven, Headed, and Screw-Threaded Fastenings, and more particularly to screws with roll-formed threads. Still more specifically, the screws herein are of the self-threading type capable of penetrating a workpiece to form their own opening, and thereafter enlarging and threading that opening to make a firm engagement therewith. The type of screws defined herein may be most useful in the field of dry-wall construction, or in joining together two sheets of metal material, but are not limited thereto.
2. Description of the Prior Art
The self-threading screws of the prior art most generally comprise a cylindrical shank portion formed on one end with an enlarged integral head and at the other end with a tapered point. One or more parallel, helical threads are formed, by rolling or cutting, on the external surface of the shank portion and extend down to the tip of the point. The external diameter of the threads increases from the tip up the tapered portion of the point, and remains uniform along the length of the shank portion.
A typical application for such a screw is the attachment of a sheet of Gypsum board to upright metal or wooden studs. After penetrating the board, which is relatively soft, the tip of the screw contacts and penetrates the stud. The threads formed on the point engage the periphery of the opening made by the tip and drive the screw further into the stud. the tapered point and expanding threads cause the opening to enlarge further as the screw advances until the opening is of the same diameter as the shank portion.
The stud or workpiece offers resistance to penetration by the point of the screw and this resistance increases as the conical point advances. This increase in resistance is due to two factors: first, the material of which the workpiece is comprised frequently workhardens as the screw penetrates; and second, as the diameter of the opening increases, the torque radius for frictional resistance increases correspondingly. The combined effect of this resistance might cause binding, or stripping of the threads on the screw or of the threads formed within the workpiece opening, or otherwise damage the screw.